The present invention relates to power management and, more particularly, to managing the peak load drawn by a site owner on an electric utility to reduce the site owner's utility bills.
Utility bills often include a peak load component that assesses a site owner a charge based on the peak usage of grid power at the site during a multi-day billing period. The multi-day billing period is typically about 30 days, but may be up to a year. The peak load is typically assessed as the maximum average load in any 30 or 60 minute period within the multi-day billing period.
To reduce the amount of the peak load charge, it is known to deploy a peak load management system that continually measures load at the site throughout the billing period and limits the use of grid power to a target peak load. The target peak load is often statically configured for the billing period, either manually or by an algorithm running on the management system.
FIG. 1 is a load-time diagram that illustrates peak load management by a conventional peak load management system, wherein the x-axis is time of day, the left y-axis is load in kilowatts and the right y-axis is battery discharge in kilowatts. Grid load 110 is the net load drawn on the electric utility that is used by the utility in calculating monthly charges. Actual load 120 is a sum of grid load 110 and stored power 130 discharged by a battery system. Actual load 120 represents the load drawn by electricity-using devices at the site, not counting the impact of the battery system, and can be considered the load profile of the site had no load management battery system been installed. At some sites, actual load 120 may be reduced by power generated at the site (e.g., output from a solar generator). Whenever during the billing period actual load 120 exceeds a target peak load 140, the management system discharges stored power 130 from the battery system (i.e., stored power 130 turns positive) to keep grid load 110 from exceeding target peak load 140. Once actual load 120 falls below target peak load 140, the management system stops discharging stored power 130 from the battery system and the battery system may be recharged (i.e., stored power 130 may turn negative).
For a conventional peak load management system to perform optimally, it must have accurate data on actual load at the site. Otherwise, the management system may discharge stored power from the battery system too early or too late, resulting in an elevated peak load and reduced cost savings. For example, FIG. 2 is a load-time diagram showing peak load management error caused by failure to account for power generated at the site (e.g., output from a solar generator) when determining actual load. The true actual load 230 is the sum of the grid load 220 and the stored power 240 discharged by a battery system, offset by the power generated at the site. However, due to failure to account for power generated at the site, the management system sees an erroneous actual load 210 that grossly overestimates true actual load 230. Such a failure might result from malfunction of a meter that measures power generation at the site or of a communications channel between such a meter and a peak load management controller. Regardless of the source of the malfunction, the effect of failure to account for power generated at the site is to overestimate actual load, which causes battery power to be discharged before true actual load 230 rises above a target peak load 250. The premature discharge of battery power results in exhaustion of battery resources before true actual load 230 falls back below target peak load 250, causing grid load 220 to spike well above target peak load 250.
In another example, FIG. 3 is a load-time diagram showing peak load management error caused by failure to account for all power consumed at the site when determining site load. The true actual load 310 is the sum of the grid load 320 and the stored power 340 discharged by a battery system offset by the power generated at the site. However, due to failure to account for all power consumed at the site, the management system sees an erroneous actual load 330 that grossly underestimates true actual load 310. Such a failure might result from malfunction of a meter that measures power consumption at the site or a communications channel between such a meter and a peak load management controller. Regardless, the effect of failure to account for all power consumed at the site is to underestimate true actual load 310, which causes battery power discharge to be postponed until after true actual load 310 rises above a target peak load 350. The belated discharge of battery power results in grid load 320 rising to a level well in excess of target peak load 350 before the discharge of battery power starts to dampen grid load 320.
In other examples, a catastrophic failure in determining actual load at the site may occur that prevents the peak load management system from making an even remotely informed decision about when to discharge battery power.
In any of these circumstances, the lack of accurate data on actual load at the site can quickly decimate the cost savings that would otherwise be achieved by the peak load management system.